A) Field of the Invention
The present invention relates to a tunneling magnetoresistance device and its manufacture method, and more particularly to a tunneling magnetoresistance device which changes its electric resistance depending on an external magnetic field and is applied to a reproducing head of a magnetic recording apparatus and a magnetic memory, and its manufacture method.
B) Description of the Related Art
In a junction having a “metal/insulating film/metal” structure consisting of the insulating film and metal films sandwiching the insulating film therebetween, as a voltage is applied across opposing metal layers, a small current flows if the insulating film is sufficiently thin. Generally, current does not flow through an insulating film. However, if the insulating film is sufficiently thin, e.g., several nm or thinner, electrons transmit through the insulating film at some probability because of the quantum mechanics effects. Current of electrons transmitting through an insulating film is called a “tunnel current” and its structure is called a “tunnel junction”.
Generally, a metal oxide film is used as the insulating film of the tunnel junction. For example, a thin insulating film of aluminum oxide is formed by natural oxidation, plasma oxidation or thermal oxidation of a surface layer of aluminum. By controlling oxidation conditions, an insulating film can be formed which is applicable to the tunnel junction and has a thickness of several nm.
A device having a tunnel junction exhibits nonlinear current-voltage characteristics and has been used as a nonlinear device.
The structure of the tunnel junction whose opposing metal layers are made of ferromagnetic material is called a “ferromagnetic tunnel junction”. A tunnel probability (tunnel resistance) of a ferromagnetic tunnel junction is dependent upon a magnetization state of opposing ferromagnetic materials. Therefore, the tunnel resistance can be changed by controlling the magnetization state by applying an external magnetic field. A tunnel resistance R can be expressed by the following equation:R=Rs+0.5ΔR(1−cos θ)where θ is a relative angle between magnetization directions of opposing ferromagnetic materials. Rs represents a tunnel resistance at the magnetization direction relative angle θ of 0, i.e., at parallel magnetization directions, and ΔR represents a difference between tunnel resistances at the magnetization direction relative angle θ of 180°, i.e., at counter-parallel magnetization directions and the tunnel resistance at the parallel magnetization directions.
A phenomenon that a tunnel resistance changes depending on a magnetization direction of ferromagnetic material results from polarization of electrons in ferromagnetic material. Generally, there exist in metal, spin-up electrons in an upward spin state and spin-down electrons in a downward spin state. There exist in nonmagnetic metal, the same number of spin-up electrons and spin-down electrons. Therefore, no magnetism is exhibited as a whole. In ferromagnetic material, the number of spin-up electrons (Nup) is different from the number of spin-down electrons (Ndown) so that the ferromagnetic material exhibits spin-up or spin-down magnetism as a whole.
It is known that when an electron transmits through a barrier layer by the tunnel phenomenon, the spin state of the electron is retained. Therefore, if there is a vacant electron quantum level in a tunnel destination ferroelectric material, an electron can transmit through the barrier layer. If there is no vacant electron quantum level, an electron cannot transmit through the barrier layer.
A change rate ΔR/Rs of a tunnel resistance is expressed by the following equation:ΔR/Rs=2P1P2/(1−P1P2)wherein P1 and P2 are spin polarizabilities of ferroelectric material on both sides of a barrier layer. The spin polarizability is given by the following equation:P=2(Nup−Ndown)/(Nup+Ndown)
Tunneling magnetoresistance devices are reported in “Japanese Patent Publication No. 2871670”, “Yuasa et al., Nature Materials vol. 3 (2004) p. 868-p. 871”, “Parkin et al., Nature Materials vol. 3 (2004) p. 862-p. 867”, and “Tsunekawa et al., Effect of Capping Layer Material on Tunnel Magnetoresistance in CoFeB/MgO/CoFeB magnetic Tunnel Junctions, International Magnetic Conference 2005, HP-08, p. 992”.